A honeycomb structure has been broadly used in a filter, a catalyst carrier, and the like, especially frequently used as a catalyst carrier in a catalytic converter for purifying an exhaust gas of an internal combustion engine such as an automobile engine, as a filter for purifying an exhaust gas of a diesel engine, and the like.
When the honeycomb structure is used in the catalyst carrier in the catalytic converter for purifying the exhaust gas of an automobile, exhaust gas regulations have tended to be strengthened year by year out of considerations to environmental problems, and there has been a demand for enhancement of a purification performance of an exhaust gas purifying catalyst so as to cope with the problems. On the other hand, from the aspect of engine development, a direction of low fuel consumption with high output is remarkably indicated, and there has also been a demand for reducing pressure loss in an exhaust gas purifying catalyst in order to cope with this situation.
Therefore, to solve the problem, trends are growing toward reduction of the pressure loss by further thinning partition walls and an outer wall of the honeycomb structure to improve breathability and toward enhancement of the purification performance at the time of warming-up by reducing a weight of the exhaust gas purifying catalyst to reduce a heat capacity.
In such applications, the honeycomb structure is used by being held in a metal can or the like on contact with a holding material. In such use, the honeycomb structure need be held at certain strength so that there occurs no slippage between the honeycomb structure and the can. Thus an isostatic strength capable of withstanding the above strength is required. Inhibition of breakage when the structure is held by the can on contact with the holding material (hereinafter referred to as the canning) is also required. Therefore, it is necessary to suppress a decrease in isostatic strength and/or a decrease in resistance to the canning (hereinafter referred to as the canning strength) when the walls of the honeycomb structure become thinner by a demand for the reduction of the heat capacity. As means for solving the strength decrease of the honeycomb structure attendant on the thinning of the partition walls, a structure in which the thickness of the partition walls is regularly reduced toward a center of a cross section of a honeycomb carrier has been proposed for example, in JP-B-54-110189. Moreover, a structure in which a cell partition walls of an outer peripheral portion is formed to be thicker than that inside has been proposed in JP-A-54-150406 and JP-A-55-147154. However, the decrease in isostatic strength attendant on the further thinning of the partition walls cannot sufficiently be suppressed even by these structures, and the decrease in canning strength cannot effectively be suppressed.
Further in WO 98/05602 was proposed a ceramic honeycomb structure wherein the average cell wall thickness T is 0.05 to 0.13 mm, the average outer wall thickness is larger than T, W>T (W is an average width of contact between cell walls and outer wall), and 0.7≧−(T/4)+0.18. However, although a certain effect is fulfilled in prevention of chipping at the time of handling in this ceramic honeycomb structure, the isostatic strength is not enhanced sufficiently, and the decrease in canning strength cannot effectively be suppressed.